18 THE VOYAGE OF H.M.S. CHALLENGER. 



quite different in all the other Siphonophorse — the Siphonanthse. Even their primary 

 medusoid larva (Siphonula = Protomeda) always possesses only a single tentacle, and 

 this is excentrically attached to the base of the primary siphon. The phylogenetic 

 cause of this peculiar position is to be sought in the bilateral modification, which the 

 older four-rayed medusoid ancestors of this legion (Codonidte) have undergone through 

 the degeneration of three marginal tentacles (Euphysidse). Only a single tentacle 

 remained persistent, and was all the more strongly developed ; it shifted in consequence 

 of the ventral splitting of the umbrella to the inferior surface of the latter beside the 

 base of the siphon. A similar centripetal migration of tentacles occurs also in Medusae, 

 sometimes on the superior, sometimes on the inferior surface of the umbrella. The 

 fact that each individual siphon of the polygastric Siphonanths bears at its base only a 

 single tentacle is simply to be regarded as the (hereditary) result of a metameric repeti- 

 tion of the primary condition. 



Only in a few Siphonophorse do the tentacles remain simple cylindrical filaments, 

 as in the Velellidee, in Apolemia, Stephalia, Linophysa, Salacia, and allied genera. 

 In the great majority a row of lateral branches (side-filaments, accessory tentacles, 

 secondary filaments, or Tentilla) is formed, as in the Cladonernidaa (Pteronema, 

 Gemmaria). The terminal equipment of the latter by manifold cnidonodes or " stinging 

 knobs" is often very characteristic of the several genera and species. The Porpitidse 

 are distinguished by the possession of three longitudinal rows of secondary branches. 

 Some supporters of the poly -person theory regard each of these accessory organs as an 

 autonomous person, and Claus even states that the mantle-like covering (involucrum) 

 which in many Physonectae surrounds each stinging knob, apparently corresponds 

 morphologically to the disc of the Medusa (1878). In my opinion all these appendages, 

 however complex their structure, are merely subordinate organs of the second order. 



PALPACLES or TASTING FILAMENTS. 



Under this designation I include only the long, extremely contractile, hollow, simple 

 filaments, which occur in the majority of Physonectae at the base of the j>alpons. They 

 have the same morphological and genetic relation to the latter as the tentacles to the 

 siphons. While the tentacles are especially of importance as capturing organs and as 

 weapons, the palpacles act as fine, far reaching, tasting organs auxiliary to the sensory 

 function of the palpons. Each palpon bears always only a single palpacle, and this 

 is always simple, never stalked. . While the supporters of the poly-person theory regard 

 each palpacle as an independent " individual," that is as a distinct person, I must on the 

 other hand agree with the poly-organ theory, which explains them simply as subordinate 

 organs of the palpons. 



